This invention relates to an interface circuit including a capacitive piezoelectric combustion pressure sensor for a location of peak pressure (LPP) ignition control in an internal combustion engine and particularly to such a circuit presenting a low impedance to the sensor output.
LPP ignition controls have been described which use sensors to generate a signal indicating combustion pressure and include apparatus for determining the occurrence of peak combustion pressure, relating this occurrence to a reference crankshaft rotational position and adjusting ignition timing in closed loop control to maintain the location of peak combustion pressure at a substantially constant predetermined crankshaft rotational position.
Such controls require combustion pressure sensors; and the most useful such sensors developed to date generally include a piezoelectric element as the transducer from pressure or strain to an electric voltage. Generally, a cable is required to conduct the electric output signal of the sensor to the location of the ignition control; and this cable extends through the environment of the vehicle engine compartment, which is an environment rich in sources of radio frequency interference (RFI). If the interface circuit for the sensor presents a high output impedance to the sensor, the cable becomes an antenna to pick up the RFI and degrade the signal to noise ratio of the sensor signal. This is particularly critical in the case of an LPP sensor and system, since it is necessary to detect the occurrence of a peak voltage. The high frequencies of the RFI make peak detection very difficult.
However, just decreasing the impedance of a resistive feedback buffer amplifier is not a practical solution, since it also decreases the bandwidth of the amplifier and thus allows phase shift of the higher frequencies in the combustion pressure signal voltage. Such phase shift is not permissible in a system for detecting the timing or phase of a peak signal voltage, since it shifts the apparent peak location. Therefore, the low impedance is obtained by a capacitive feedback amplifier similar to a charge amplifier. However, it is also important to preserve the low end of the bandwidth down to 0.5 Hz. The large storage and filter capacitor required to maintain a constant DC operating level and still treat 0.5 Hz as AC takes time to charge when the system starts, as does the large DC blocking capacitor required between the sensor and amplifier input because of the sensor's own capacitance.